Metals and metallurgy

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1. Metals and M etallurgical P rinciples:

2. Characteristics of metals, non-metals and metalloids : Elements are classified into metals, non-metals and metalloids.   1. Metals: All elements except hydrogen , which form positive ions by losing electrons during chemical reactions are called metals . Thus metals are electropositive elements. They are characterized by bright lustre, hardness, and are excellent conductors of heat and electricity. Metals are solids under normal conditions except for Mercury. They are ductile (can be drawn into wire) and malleable (can be beaten into very thin sheets). 2. Non-metals: Elements that tend to gain electrons to form anions during chemical reactions are called non-metals. These are electronegative elements. They are non-lustrous, brittle and poor conductors of heat and electricity (except graphite). Non-metals can be gaseous, liquids or solids. 3. Metalloids: Elements that behave like both metals and non-metals are called metalloids .

3. Common metal, non-metals and metalloids: Metals Non-metals Metalloids Gold Oxygen Silicon Silver Carbon Boron Copper Hydrogen Arsenic Iron Nitrogen Antimony Mercury Sulphur Germanium Zinc Phosphorus

4. Minerals and ores: The natural elements in which the metals or their compounds occur in the earth are called minerals . Ores are minerals from which metals are conveniently and economically extracted. All the ores are minerals, but all the minerals are not ores. For example , iron is found in the crust of the earth as oxides, carbonates and sulphides. Out of these minerals of iron, oxides of iron are employed in the extraction of the metal. Thus oxides of iron are called ores of the iron .

5. For example, Minerals of Sodium : Albite (soda feldspar) -NaAlSi 3 O 8 Borax - Na 2 B 2 O 7 .10H 2 O Glauber's salt- Na 2 SO 4 .10H 2 O Sodium chloride, (common salt) found as rock salt, in sea water and in lakes. Sodium nitrate- (NaNO 3 ) as chile saltpetre. Out of these minerals only NaCl has been used economically to extract metal and hence NaCl is an ore of sodium.

6. Different process involved in metallurgical process: The various processes involved in the extraction of metals from their ores and refining are known as metallurgy . Different process involved in metallurgical process The actual process of extraction of a metal from its ore depends upon the nature of the ore and the metal. There is no universally operational method for the extraction of metals. Metallurgy is of three types

7. A . Pyro metallurgy : – It is the method of extraction of metals from the ore by chemical reduction at very high temperature. B. Electro metallurgy :- I t is the method of extraction of metals from the ore by electrolytic reduction in molten state or in aqueous solution. C. Hydro metallurgy : – It is the method of extraction of metals from the ore by dissolving the ore with suitable reagent and subsequent precipitation of the metal by other active metal.

8. Steps involved in a metallurgical process The extraction of a metal from its ore involves the following steps: a. Mining of ore: Most ores generally occur deep inside the Earth. Some may occur only a few metres under the earth's surface. 'Mining' is the process of taking out the ores from the mines. When an ore occurs near the surface of the Earth, it can be directly dug out. b. Crushing of the ore: Extracted ore often occurs in big lumps. It is essential to break it into smaller pieces. The lumps are crushed to smaller pieces by hammering in a hammer mill or by help of a jaw-crusher. c. Grinding and pulverization of the crushed ore: The crushed ore is then finally pulverized to fine powder state in a stamp mill or a pulveriser.

9. d. Concentration of the ore (ore dressing) The removal of the undesired foreign impurities i.e., gangue, from the ore is called concentration of the ore. Either of the following methods is used for concentrating the ores: 1. Hand picking: If the impurities present are quite distinct from the ore, and are of large size, these may be removed by hand picking. This method is slow and is generally adopted in the initial stages of concentration.

10. 2. Gravity or levigation method: The gravity separation is based on the difference in the density of the ore and the impurities. This method of separation is used for the concentration of oxides ores like bauxite,Al 2 O 3 .2H 2 O, heamatite,Fe 2 O 3 etc. In this process the powdered ore is washed with running stream of water. The heavy ore particles settle down while lighter impurities are washed away( as shown in diagram). The powdered ore is dropped from the top, and a stream of water is pushed in from the bottom. The lighter gangue is washed away by water and the heavier ore particles settle down, and are removed.

11. Fig. gravity separation method

12. 3. Magnetic separation: Magnetic separation is done especially in the case of haematite ore, whereby the powdered ore is dropped on to leather or brass conveyer belt, which moves over two rollers one of these rollers, is magnetic. When the ore passes over the magnetic roller, it sticks to the belt due to the force of attraction and falls nearer due to the force of attraction of the magnetized roller. The gangue falls over readily, further away. The ore and the magnetic impurity are collected as two separate heaps.

13. Fig : - Magnetic separation

14. 4. Froth flotation process: This process is used for concentrating sulphide ores, as such ores are preferentially wetted by oil while the gangue particles are wetted by water. Powdered ore is mixed with water and a little pine oil and the mixture is vigorously stirred by passing compressed air. The froth, which is produced rises to the surface and carries the ore particles along with it. The gangue is left behind

15. Fig: - The froth flotation process

16. 5. Leaching process: In this method, the ore is treated chemically with a suitable reagent that preferentially dissolves the active component of the ore. The concentrated ore form is then recovered from the solution by a suitable chemical method. A typical example of ore concentration by leaching process is the purification of bauxite using NaOH solution as a leachant . The Bauxite is digested with concentrated solution of caustic soda at 150°C . The Aluminium oxide dissolves in NaOH leaving behind the insoluble impurities, which are removed by filtration .

17. The solution of NaAlO 2  (sodium meta-aluminate) is then treated with freshly prepared Al(OH) 3  when the entire aluminium in the solution gets precipitated as Al(OH) 3 The precipitate of Al(OH) 3  is removed, washed and dried to get Al 2 O 3 .

18. e. Calcination : The concentrated ore is converted into oxide by calcination i.e., heating it strongly in the absence of air or roasting (heating it strongly in presence of air). This helps in removing volatile impurities like CO 2 , SO 2 , organic matter, and moisture from the ore. For example, It removes moisture from bauxite .

19. i t removes CO 2  from carbonate ores e.g.,

20. f. Roasting : In this process the ore (usually sulphide) is heated strongly, in the presence of excess of air but below its melting temperature. The result is It removes moisture, CO 2 , SO 2  and organic matter. The sulphide ore is converted partly into its oxide.

21. g. Reduction: In this process the metal oxide obtained from roasting or calcination is turned into free metal by the reduction with different reagents according to the nature of the ore. 1. Carbon reduction (smelting) : It is a process in which carbon used for the reduction of fused metal oxide into free metals. This method is used for the extraction of metals like iron, copper, zinc, tin. In this process roasted or calcined ore is mixed with suitable quantity of coke or charcoal and heated to a very high temperature. This method is also called as smelting . This process is carried mainly out in a blast furnace in a controlled supply of air.

22. ZnO + C Zn + CO SnO 2 + 2C Sn + 2CO PbO + C Pb + CO Fe 2 O 3 + 3C 2Fe + 3CO During reduction, an additional reagent is also added to the ore to remove the impurities still present in the ore. This additional reagent is called  flux . Flux combines with the impurities to form a fusible product called  slag . Flux + Impurities → Slag  

23. The selection of flux depends upon the nature of impurities. If impurities are  acidic  in nature, the flux is basic, lime ( CaO ). On the other hand, for basic impurities, are acidic flux such as silica (SiO 2 ) is used. CaO     +    SiO 2  → CaSiO 3

24. Differences between flux and slag flux slag Flux is a chemical substance that is added along with carbon during smelting to remove infusible impurities in order to form the fusible mass. It may be acidic or basic. . Example of flux are CaO , FeO , SiO 2   Slag is a chemical substance that is formed by the combination of the infusible impurities present in the ore and a flux which is lighter than molten metal. It is neutral compound. Example of flux are CaSiO 3  , FeSiO 3  .

25. Note: Slag is important in metallurgy because being fusible mass it helps to remove infusible impurities from the concentrated ores during metallurgy.

26. 3. Aluminothermy process: The process of reduction of a metal oxide to the metal with the help of aluminium powder as the reducing agent is called (aluminothermy or Goldschmidt) thermite process. Aluminium is used as a reducing agent for the extraction of chromium, manganese from their respective oxides because aluminium is more electro positive then chromium and manganese. 2Al + Cr 2 O 3 Al 2 O 3 + 2Cr 8Al + 3Mn 3 O 4 Al 2 O 3 + 9Mn  

27. Refining of metals : Purification of the metal is the last step in metallurgy. Refining is based on the difference between the properties of metals and their impurities.   i . Liquation This method is used for refining metals having a low melting point. e.g., tin, lead, bismuth

28. Process: The metal to be refined is placed over the sloping hearth of a furnace The temperature of the furnace is maintained slightly above the melting point of the metal Pure metal melt and flow down Impurities, having higher melting point, are left behind

29. Distillation: This method is used for volatile metals having boiling points lower than their impurities. e.g., zinc, mercury. Process The metal to be refined is heated above its boiling point Impurities do not vaporise Pure metal vaporises and is condensed Impurities are left behind

30. Poling : Oxide impurity of impure metals can be purified by this method. Poles of green wood are used to stir the molten crude metal sample. The hydrocarbon organic matter in the poles acts as a reducing agent and reduces the oxide impurities. This method is used for refining copper. 2Cu 2 O + CH 4 8Cu + CO 2 + 2H 2 O  

31. Electrolytic method: A large number of metals e.g., copper, silver, gold, nickel etc. are refined by this method. In an electrolytic cell the impure metal is made anode and a thin strip of pure metal is made cathode. A solution of a suitable salt of the concerned metal is used to fill the electrolytic cell. On passing electricity, the anode undergoes dissolution while the pure metal gets deposited at cathode. Insoluble impurities fall below the anode in the form of anode mud.

32. Fig : - Electrolytic purification of a metal